From: Oleg Nesterov <oleg-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> To: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> Cc: Frederic Weisbecker <fweisbec-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>, Containers <containers-cunTk1MwBs9QetFLy7KEm3xJsTq8ys+cHZ5vskTnxNA@public.gmane.org>, LKML <linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Tejun Heo <tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>, Cgroups <cgroups-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>, Andrew Morton <akpm-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.org>, "Paul E. McKenney" <paulmck-23VcF4HTsmIX0ybBhKVfKdBPR1lH4CV8@public.gmane.org>, Paul Menage <paul-inf54ven1CmVyaH7bEyXVA@public.gmane.org> Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking Date: Thu, 12 Jan 2012 18:07:28 +0100 [thread overview] Message-ID: <20120112170728.GA25717@redhat.com> (raw) In-Reply-To: <20120112003102.GB9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> Hi Mandeep, On 01/11, Mandeep Singh Baines wrote: > > > > > > #define while_each_thread(g, t, o) \ > > > while (t->group_leader == o && (t = next_thread(t)) != g) > > > > > > Where o should have the value of g->group_leader. > > > > I don't understand how this helps... and how this can work even > > ignoring the barriers. > > > > OK, we have the main thream M and the sub-thread T, we are doing > > > > do { > > do_something(t); > > } while_each_thread(M, t, M); > > > > why we can't miss T if it does exec? > > > > So for: > > struct task *M; /* assuming this is passed in to us */ > struct task *L = M->group_leader; L == M > do { > do_something(T); > } while_each_thread(M, T, L); > > Here is my thinking. > > If some thread K does exec, you won't miss it because: > > 1) Ignoring the group_leader check, you'll visit K just by following > next_thread(). That's the case today and is what you except > when iterating over an rcu_list. > 2) (t->group_leader == o) will fail iff t is the exec thread. > Since we test t->group_leader before re-assigning it (t=next_thread()), > the test will fail only after visiting the exec thread. So you'll > visit the exec thread and then terminate the loop. Still can't understand... Lets look at this trivial example again. We start from the main thread M, it is ->group_leader. There is another thread T in this thread group. We are doing OLD = M; t = M; do { do_smth(t); } while (t->group_leader == OLD && ((t = next_thread(t)) != M); The first iteration does do_smth(M). T calls de_thread() and, in particular, it does M->group_leader = T (see "leader->group_leader = tsk" in de_thread). after that t->group_leader == OLD fails. t == M, its group_leader == T. do_smth(T) won't be called. No? Oleg.
WARNING: multiple messages have this Message-ID (diff)
From: Oleg Nesterov <oleg@redhat.com> To: Mandeep Singh Baines <msb@chromium.org> Cc: Frederic Weisbecker <fweisbec@gmail.com>, Li Zefan <lizf@cn.fujitsu.com>, Tejun Heo <tj@kernel.org>, LKML <linux-kernel@vger.kernel.org>, Containers <containers@lists.linux-foundation.org>, Cgroups <cgroups@vger.kernel.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Paul Menage <paul@paulmenage.org>, Andrew Morton <akpm@linux-foundation.org>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> Subject: Re: Q: cgroup: Questions about possible issues in cgroup locking Date: Thu, 12 Jan 2012 18:07:28 +0100 [thread overview] Message-ID: <20120112170728.GA25717@redhat.com> (raw) In-Reply-To: <20120112003102.GB9511@google.com> Hi Mandeep, On 01/11, Mandeep Singh Baines wrote: > > > > > > #define while_each_thread(g, t, o) \ > > > while (t->group_leader == o && (t = next_thread(t)) != g) > > > > > > Where o should have the value of g->group_leader. > > > > I don't understand how this helps... and how this can work even > > ignoring the barriers. > > > > OK, we have the main thream M and the sub-thread T, we are doing > > > > do { > > do_something(t); > > } while_each_thread(M, t, M); > > > > why we can't miss T if it does exec? > > > > So for: > > struct task *M; /* assuming this is passed in to us */ > struct task *L = M->group_leader; L == M > do { > do_something(T); > } while_each_thread(M, T, L); > > Here is my thinking. > > If some thread K does exec, you won't miss it because: > > 1) Ignoring the group_leader check, you'll visit K just by following > next_thread(). That's the case today and is what you except > when iterating over an rcu_list. > 2) (t->group_leader == o) will fail iff t is the exec thread. > Since we test t->group_leader before re-assigning it (t=next_thread()), > the test will fail only after visiting the exec thread. So you'll > visit the exec thread and then terminate the loop. Still can't understand... Lets look at this trivial example again. We start from the main thread M, it is ->group_leader. There is another thread T in this thread group. We are doing OLD = M; t = M; do { do_smth(t); } while (t->group_leader == OLD && ((t = next_thread(t)) != M); The first iteration does do_smth(M). T calls de_thread() and, in particular, it does M->group_leader = T (see "leader->group_leader = tsk" in de_thread). after that t->group_leader == OLD fails. t == M, its group_leader == T. do_smth(T) won't be called. No? Oleg.
next prev parent reply other threads:[~2012-01-12 17:07 UTC|newest] Thread overview: 73+ messages / expand[flat|nested] mbox.gz Atom feed top 2011-12-21 3:43 Q: cgroup: Questions about possible issues in cgroup locking Frederic Weisbecker 2011-12-21 3:43 ` Frederic Weisbecker 2011-12-21 13:08 ` Oleg Nesterov 2011-12-21 13:08 ` Oleg Nesterov 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 19:01 ` Mandeep Singh Baines 2011-12-21 19:01 ` Mandeep Singh Baines [not found] ` <20111221190102.GE13529-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2011-12-21 19:08 ` Frederic Weisbecker 2011-12-21 19:08 ` Frederic Weisbecker 2011-12-21 19:24 ` Mandeep Singh Baines 2011-12-21 19:24 ` Mandeep Singh Baines [not found] ` <20111221192413.GF13529-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-21 20:04 ` Frederic Weisbecker 2011-12-22 15:30 ` Oleg Nesterov 2011-12-22 15:30 ` Oleg Nesterov [not found] ` <20111222153004.GA30522-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-04 19:36 ` Mandeep Singh Baines 2012-01-04 19:36 ` Mandeep Singh Baines 2012-01-04 19:36 ` Mandeep Singh Baines [not found] ` <20120104193614.GF9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-06 15:23 ` Oleg Nesterov 2012-01-06 15:23 ` Oleg Nesterov [not found] ` <20120106152356.GA23995-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-06 18:25 ` Mandeep Singh Baines 2012-01-06 18:25 ` Mandeep Singh Baines 2012-01-06 18:25 ` Mandeep Singh Baines [not found] ` <20120106182535.GJ9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-11 16:07 ` Oleg Nesterov 2012-01-11 16:07 ` Oleg Nesterov [not found] ` <20120111160730.GA24556-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-12 0:31 ` Mandeep Singh Baines 2012-01-12 0:31 ` Mandeep Singh Baines [not found] ` <20120112003102.GB9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-12 17:07 ` Oleg Nesterov [this message] 2012-01-12 17:07 ` Oleg Nesterov 2012-01-12 17:57 ` Mandeep Singh Baines 2012-01-12 17:57 ` Mandeep Singh Baines [not found] ` <20120112175725.GD9511-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-13 15:20 ` Oleg Nesterov 2012-01-13 15:20 ` Oleg Nesterov [not found] ` <20120113152010.GA19215-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-13 18:27 ` Mandeep Singh Baines 2012-01-13 18:27 ` Mandeep Singh Baines [not found] ` <20120113182750.GD18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-14 17:36 ` Oleg Nesterov 2012-01-14 17:36 ` Oleg Nesterov 2012-01-18 23:17 ` Mandeep Singh Baines 2012-01-18 23:17 ` Mandeep Singh Baines [not found] ` <20120118231742.GS18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-19 15:45 ` Oleg Nesterov 2012-01-19 15:45 ` Oleg Nesterov 2012-01-19 18:18 ` Mandeep Singh Baines 2012-01-19 18:18 ` Mandeep Singh Baines [not found] ` <20120119181803.GU18166-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-01-20 15:06 ` Oleg Nesterov 2012-01-20 15:06 ` Oleg Nesterov 2012-03-20 19:34 ` Oleg Nesterov 2012-03-20 19:34 ` Oleg Nesterov [not found] ` <20120320193414.GA21277-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-03-21 18:59 ` Mandeep Singh Baines 2012-03-21 18:59 ` Mandeep Singh Baines 2012-03-23 17:51 ` Oleg Nesterov 2012-03-23 17:51 ` Oleg Nesterov [not found] ` <20120321185955.GK27051-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> 2012-03-23 17:51 ` Oleg Nesterov [not found] ` <20120119154522.GA14058-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-19 18:18 ` Mandeep Singh Baines 2012-03-20 19:34 ` Oleg Nesterov [not found] ` <20120114173648.GA32543-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-18 23:17 ` Mandeep Singh Baines [not found] ` <20120112170728.GA25717-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2012-01-12 17:57 ` Mandeep Singh Baines 2011-12-22 15:30 ` Oleg Nesterov 2011-12-21 19:01 ` Mandeep Singh Baines 2012-02-01 16:28 ` Frederic Weisbecker 2012-02-01 16:28 ` Frederic Weisbecker [not found] ` <20111221130848.GA19679-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2011-12-21 17:56 ` Frederic Weisbecker 2011-12-21 17:59 ` Frederic Weisbecker 2011-12-21 17:59 ` Frederic Weisbecker 2011-12-21 18:11 ` Oleg Nesterov 2011-12-21 18:11 ` Oleg Nesterov [not found] ` <20111221181101.GA3092-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> 2011-12-21 18:23 ` Frederic Weisbecker 2011-12-21 18:23 ` Frederic Weisbecker 2011-12-21 18:23 ` Frederic Weisbecker 2012-02-01 16:28 ` Frederic Weisbecker 2011-12-21 13:08 ` Oleg Nesterov -- strict thread matches above, loose matches on Subject: below -- 2011-12-21 3:43 Frederic Weisbecker
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